Method and apparatus for detecting AFE filter capacitor degradation

1. A power conversion system, comprising: an active front end rectifier, comprising a multiphase AC input, and a plurality of switching devices operative according to a plurality of rectifier switching control signals to convert power received at the multiphase AC input to provide DC output power; an input filter circuit coupled between a power converter input and the active front end rectifier, the input filter circuit comprising a plurality of filter inductors coupled between a corresponding phase of the power converter input and a corresponding phase of the multiphase AC input of the active front end rectifier, and a capacitor circuit including a plurality of filter capacitors individually connected to at least one of the plurality of filter inductors; a sensing circuit operative to determine a plurality of measured currents and measured voltages associated with the input filter circuit; and a degradation detection system operatively coupled with the input filter circuit and comprising at least one processor configured to: compute fundamental frequency current and voltage phasors based on the measured currents and voltages, compute positive and negative sequence current and voltage component phasors based on the computed fundamental frequency current and voltage phasors, compute at least one off-diagonal term of a filter circuit admittance matrix based on at least one of the computed positive and negative sequence current and voltage component phasors, compare the at least one off-diagonal term to at least one threshold, and selectively detect degradation of one or more of the filter capacitors if the at least one off-diagonal term exceeds the at least one threshold.

2. The power conversion system of claim 1 , wherein the sensing circuit is operative to determine a plurality of measured capacitor bank branch line currents and measured line-line voltages associated with the input filter circuit, and wherein the at least one processor is configured to compute the fundamental frequency current and voltage phasors based on the plurality of measured capacitor bank branch line currents and measured line-line voltages.

3. The power conversion system of claim 1 , wherein the at least one processor is configured to compute an adjusted off-diagonal term by offsetting the at least one off-diagonal term of the filter circuit admittance matrix based on a predetermined nominal asymmetry value, to compare the adjusted off-diagonal term to the at least one threshold, and to selectively detect degradation of one or more of the filter capacitors if the adjusted off-diagonal term exceeds the at least one threshold.

4. The power conversion system of claim 1 , wherein the at least one processor is configured to compute the at least one off-diagonal term of the filter circuit admittance matrix as an off-diagonal term of the filter circuit admittance matrix representing an effect of positive sequence voltage on negative sequence current in the input filter circuit.

5. The power conversion system of claim 4 , wherein the at least one processor is configured to compute the off-diagonal term Ŷ np of the filter circuit admittance matrix representing the effect of positive sequence voltage on negative sequence current in the input filter circuit based on a positive sequence current phasor Ĩ p , a negative sequence current phasor Ĩ n , a positive sequence voltage phasor {tilde over (V)} p , and a negative sequence voltage phasor {tilde over (V)} n according to the following formula: Y ^ np = I ~ n ⁢ I ~ p V ~ p ⁢ V ~ n V ~ p .

6. The power conversion system of claim 1 , wherein the at least one processor is configured to: determine individual impedance values for the plurality of filter capacitors, and to identify at least one particular filter capacitor as being suspected of degradation based on the corresponding determined impedance value.

7. The power conversion system of claim 6 : wherein the plurality of filter capacitors are connected in a delta configuration; wherein the at least one processor is configured to determine individual impedance values for the plurality of filter capacitors by: computing line-neutral admittance matrix elements and line-line positive and negative sequence current and voltage component phasors based on line-neutral positive and negative sequence current and voltage component phasors, computing line-line admittance matrix components based on the line-neutral admittance matrix components, computing a zero sequence circulating current phasor based on at least some of the line-line admittance matrix components, and computing individual Delta line-line impedance values for at least some of the filter capacitors; and wherein the at least one processor is configured to identify at least one particular filter capacitor as being suspected of degradation by: comparing individual Delta line-line impedance values with the at least one threshold value, and identifying at least one of the filter capacitors as being suspected of degradation if a corresponding Delta line-line impedance value exceeds the at least one threshold value.

8. The power conversion system of claim 6 : wherein the plurality of filter capacitors are connected in a Y configuration; wherein the at least one processor is configured to determine individual impedance values for the plurality of filter capacitors by: computing a line-neutral admittance matrix based on line-neutral positive and negative sequence current and voltage component phasors, computing a line-neutral impedance matrix by inverting the line-neutral admittance matrix, computing line-neutral positive and negative sequence voltage component phasors based online-line positive and negative sequence voltage component phasors, computing a neutral zero sequence voltage phasor based on at least some line-neutral impedance matrix components and the computed line-neutral positive and negative sequence voltage component phasors, and computing individual Y line-neutral impedance values for at least some of the filter capacitors based on the line-neutral positive and negative sequence current and voltage component phasors and the computed neutral zero sequence voltage phasor; and wherein the at least one processor is configured to identify at least one particular filter capacitor as being suspected of degradation by: comparing the individual Y line-neutral impedance values with the at least one threshold value, and identifying at least one of the filter capacitors as being suspected of degradation if a corresponding Y line-neutral impedance values exceeds the at least one threshold value.

9. A method for detecting filter capacitor degradation in an input filter of an active front end power conversion system, the method comprising: determining a plurality of measured currents and measured voltages associated with the input filter circuit; computing fundamental frequency current and voltage phasors based on the measured currents and voltages; computing positive and negative sequence current and voltage component phasors based on the computed fundamental frequency filter current and voltage phasors; computing at least one off-diagonal term of a filter circuit admittance matrix based on at least one of the computed positive and negative sequence current and voltage component phasors; comparing the at least one off-diagonal term to at least one threshold; and selectively detecting degradation of one or more of the filter capacitors if the at least one off-diagonal term exceeds the at least one threshold.

10. The method of claim 9 , comprising: computing an adjusted off-diagonal term by offsetting the at least one off-diagonal term for of the filter circuit admittance matrix based on a predetermined nominal asymmetry value; comparing the adjusted off-diagonal term to the at least one threshold; and selectively detecting degradation of one or more of the filter capacitors if the adjusted off-diagonal term exceeds the at least one threshold.

11. The method of claim 9 , comprising: determining individual impedance values for the plurality of filter capacitors; and identifying at least one particular filter capacitor as being suspected of degradation based on a corresponding determined impedance value.

12. The method of claim 11 : wherein the plurality of filter capacitors are connected in a delta configuration; wherein determining the individual impedance values for the plurality of filter capacitors comprises: computing line-neutral admittance matrix elements and line-line positive and negative sequence current and voltage component phasors based on line-neutral positive and negative sequence current and voltage component phasors, computing line-line admittance matrix components based on the line-neutral admittance matrix components, computing a zero sequence circulating current phasor based on at least some of the line-line admittance matrix components, and computing individual Delta line-line impedance values for at least some of the filter capacitors; and wherein identifying at least one particular filter capacitor as being suspected of degradation comprises: comparing individual Delta line-line impedance values with the at least one threshold value, and identifying at least one of the filter capacitors as being suspected of degradation if a corresponding Delta line-line impedance value exceeds the at least one threshold value.

13. The method of claim 11 : wherein the plurality of filter capacitors are connected in a Y configuration; wherein determining the individual impedance values for the plurality of filter capacitors comprises: computing a line-neutral admittance matrix based on line-neutral positive and negative sequence current and voltage component phasors, computing a line-neutral impedance matrix by inverting the line-neutral admittance matrix, computing line-neutral positive and negative sequence voltage component phasors based online-line positive and negative sequence voltage component phasors, computing a neutral zero sequence voltage phasor based on at least some line-neutral impedance matrix components and the computed line-neutral positive and negative sequence voltage component phasors, and computing individual Y line-neutral impedance values for at least some of the filter capacitors based on the line-neutral positive and negative sequence current and voltage component phasors and the computed neutral zero sequence voltage phasor; and wherein identifying at least one particular filter capacitor as being suspected of degradation comprises: comparing the individual Y line-neutral impedance values with the at least one threshold value, and identifying at least one of the filter capacitors as being suspected of degradation if a corresponding Y line-neutral impedance values exceeds the at least one threshold value.

14. A power conversion system, comprising: an active front end rectifier, comprising a multiphase AC input, and a plurality of switching devices operative according to a plurality of rectifier switching control signals to convert power received at the multiphase AC input to provide DC output power; an input filter circuit coupled between the power converter input and the active front end rectifier, the input filter circuit comprising a plurality of filter inductors coupled between a corresponding phase of the power converter input and a corresponding phase of the multiphase AC input of the active front end rectifier, and a capacitor circuit including a plurality of filter capacitors individually connected to at least one of the plurality of filter inductors; a sensing circuit operative to determine a plurality of measured currents and measured voltages associated with the input filter circuit; and a degradation detection system operatively coupled with the input filter circuit and comprising at least one processor configured to: compute an effective neutral voltage value based on the measured voltages, compute an effective neutral voltage mean value based on the effective neutral voltage value, compute a nominal effective neutral voltage value based on the effective neutral voltage value and the effective neutral voltage mean value, compute an effective current value based on the measured currents, compute an effective current mean value based on the effective current value, compute a nominal effective current value based on the effective current value and the effective current mean value, compute an effective current fault value based on the nominal effective current value, an estimated admittance value, and the nominal effective neutral voltage value, compare the effective current fault value to at least one threshold, and selectively detect degradation of one or more of the filter capacitors if the effective current fault value exceeds the at least one threshold.

15. The power conversion system of claim 14 , wherein the sensing circuit is operative to determine a plurality of measured capacitor bank branch line currents and measured line-line voltages associated with the input filter circuit, and wherein the at least one processor is configured to compute the effective neutral voltage value based on the measured line-line voltages and to compute the effective current value based on the measured capacitor bank branch line currents.

16. A method for detecting filter capacitor degradation in an input filter of an active front end power conversion system, the method comprising: determining a plurality of measured currents and measured voltages associated with the input filter circuit; computing an effective neutral voltage value based on the measured voltages, computing an effective neutral voltage mean value based on the effective neutral voltage value, computing a nominal effective neutral voltage value based on the effective neutral voltage value and the effective neutral voltage mean value, computing an effective current value based on the measured currents, computing an effective current mean value based on the effective current value, computing a nominal effective current value based on the effective current value and the effective current mean value, computing an effective current fault value based on the nominal effective current value, an estimated admittance value, and the nominal effective neutral voltage value, comparing the effective current fault value to at least one threshold, and selectively detecting degradation of one or more of the filter capacitors if the effective current fault value exceeds the at least one threshold.

17. The method of claim 16 , comprising: determining a plurality of measured capacitor bank branch line currents and measured line-line voltages associated with the input filter circuit; computing the effective neutral voltage value based on the measured line-line voltages; and computing the effective current value based on the measured capacitor bank branch line currents.